The present invention relates to a monitor television apparatus and, in particular, to a monitor television apparatus which executes a diversity reception technique using a plurality of antennas.
A diversity reception system using a plurality of antennas is known as one of the techniques to receive television (TV) signals while reducing the fading influence. In a TV reception system employing this diversity reception system, a TV tuner performs the diversity function for each vertical blanking period and outputs a composite video signal, using n antennas.
More specifically, after a lapse of a given time after a vertical sync signal falls, an antenna switching pulse is generated to instantly switch from the presently selected antenna to another one, which changes the level of a received signal. This changed level is compared with the previous level, and if it is smaller than the previous one, the antenna is switched to the previous antenna. On the other hand, if the changed level is greater than the previous one, the selected antenna is retained for signal reception.
The antenna is again switched to another one by the next switching pulse that is generated after one vertical period, and the signal level comparison is similarly performed. When the difference in level between the signals before and after the antenna switching is large, a large pulse appears in the composite video signal in synchronism with the switching pulse.
The composite video signal generated after video signal detection by the TV tuner is input to a demodulator of the monitor TV apparatus. The demodulator reproduces a DC component originated from the clamping of the pedestal level and performs a signal processing such as color demodulation, so as to provide the primary color signals, red (R), green (G), and blue (B) signals. Each primary signal is amplified by its associated video amplifier.
The composite video signal is also supplied to a deflection circuit of the monitor TV apparatus, which separates sync signals from the video signal, and supplies a horizontal sync signal to a phase shifter while supplying a proper blanking pulse to the demodulator. The phase shifter shifts the phase of the horizontal sync pulse to generate a clamp signal in phase with a color burst signal and supplies the clamp signal to the demodulator. The deflection circuit supplies a deflection output to drive a color cathode ray tube (CRT) which receives the amplified R, G, and B color signals to display a color image.
When the antenna switching pulse is out of phase with the clamp pulse, no trouble occurs. When these pulses are in phase, however, an undesirable pulse appears in the composite video signal in synchronism with the switching pulse, as mentioned above, and is superimposed on the clamp pulse. Since the demodulator of the monitor TV apparatus clamps the composite video signal to reproduce the DC component, the primary color signals demodulated after the clamping each have a sag portion that is deflected significantly from a normal pedestal clamp pulse. As the restoration of the sag continues over the vertical scanning period, a brightness change occurs at the upper portion on the display screen which corresponds to the beginning of the vertical scanning period.